A zigzag sewing machine of this type is disclosed in, for example, Japanese Laid-Open(Kokai) Patent Publication No. 56-145888.
As shown in FIG. 4, in this known zigzag sewing machine, the respective motions of a rotary member 9 and a shuttle 15 are timed so that the junction 13 of the rotary member 9 and a connecting rod 12, and the shuttle hook 15a of a shuttle 15 move in the same direction a that of the swinging motion of a needle bar supporting member when the shuttle hook 15a of the shuttle 15 catches a loop formed in the eye of a needle 8. A horizontal fixed shaft 4 on which the needle bar supporting member swings is disposed so that the junction 14 of a needle bar 7 and a connecting rod 12 is located below the horizontal fixed shaft 4 at a moment when the shuttle hook 15a catches a loop. Thus, the difference h1 is minimized between the height of the needle 8 at one end of its swinging motion and the height of the same at the other end of the swinging motion, where the shuttle hook 15a catches a loop, in order to enable the shuttle hook 15a to encounter the needle 8 at optimum positions.
This known zigzag sewing machine, however, needs an inverting mechanism to rotate the rotary member 9 and the shuttle 15 in opposite directions, respectively, and hence the zigzag sewing machine has a complicated construction and many component parts.